The present invention relates to an apparatus for the electro-kinetic charging of powder-like materials for the purpose of electrostatic coating of objects with such powder. It can be used in manual as well as automatically operated powder spray devices, also in fluidized bed apparatus, as a charging device. For the coating material one may use duro or thermoplastic powders, enamel or similar materials in powder form.
There are spray apparatus known for the electrostatic surface coating, in which the powdered coating material while being pneumatically forced through an insulating material channel, becomes electrically charged through the frictional effect due to the fact that the powder particles contact the wall of the flow channel (for example, German Patent No. 1,577,757 and 2,203,351).
The disadvantage of such apparatus resides in that it can be operated only with a relatively low powder output to accomplish an intensive all-around contact of the powder particles with the flow-through channel to obtain a charge which is satisfactory from the viewpoint of the technical requirement of the process. In order to increase the powder charging, various measures have been undertaken, which were intended to increase the turbulence of the flow, and to this effect special turbulence creating means have been inserted (German Laid-Open Application No. 2,938,606), also propellers, (U.S. Pat. No. 3,905,330) or a blower (German Laid-Open Application No. 2,209,231) and curved charging pipes (German Laid-Open Application No. 3,100,002). Furthermore, there are constructive variants known, in which by means of a half-toroid-shaped input section under the use of the Coanda effect (German Laid-Open Application No. 2,713,697) or by means of producing a helical-shaped particle path (German Laid-Open Application No. 2,756,009) an intensive wall contact is accomplished.
Furthermore, there are electro-kinetic charging devices known, in which the charging of the powder is accomplished by a combination of a frictional-electric effect with the ionization process caused by it on passive electrostatic electrodes (East German Patent Nos. 106,308 and 113,289 and German Laid-Open Application No. 3,303,137). According to the latter publications, the pneumatically introduced powder is undergoing a turbulence effect in an insulating material pipe caused by a gas flow. The device is in the form of a jet apparatus and includes a pair of input channels meeting in the starting section of the insulating mateial channel and, in the region of the combined gas flow, one or more electrostatic ionizing electrodes are provided. Such electrodes are in the form of rings embedded into the inner wall of the insulating material pipe or, in the form of axially or radially projecting needle electrodes projecting into the insulating pipe.
There is also a device known in which in the direction of the flow, behind or in the immediate vicinity of the electrostatic ionizing electrode, a cone-shaped flow-member is arranged for the increasing of the frictional electrical effect (East German Patent No. 134,841). All the devices which operate with an additional gas flow have one feature in common, namely, that they operate on the principle of a jet apparatus, whereby the uniform nature of the powder-gas flow introduced by a hose conduit and through an ejector, becomes effected. The devices having the electrostatic ionizing electrodes possess the disadvantage that the ionization process occurs on certain preferred points and, on these points, due to the partially high field strengths as a result of the glow discharge, the powder particles will become sintered or melted onto the electrodes, whereupon the ionization becomes reduced and, the powder charging falls-off. The electrostatic electrodes, therefore, must undergo a removal of the powder crust developed on them after certain period of time.